1. Field of the Invention
The present invention relates to a process for producing a soft blackplate with a temper of T-3 or less to be subjected to surface treatment, such as tin-plating or chromic-acid treatment. More particularly, the present invention relates to a method for producing a soft blackplate for surface treatment (below, simply blackplate) by continuous annealing without the aid of decarburization by vacuum-degassing during the steel-making and without additive elements such as Ti, Nb. Still more particularly the present invention relates to a method for the producing a blackplate exhibiting soft properties and improved fluting resistance.
2. Description of the Related Art
The "temper degree" is an index defined by Japan Industrial Standard (JIS) G 3303 enabling selection of a blackplate for surface treatment, such as tin plating, having the desired material properties. The temper degree is expressed in terms of Rockwell superficial hardness (H.sub.R 30T or H.sub.R 15T) with T-1: 46 to 52; T-2: 50 to 56; T-3: 54 to 60; T-4: 58 to 64; T-5: 62 to 68; and T-6: 67 to 73 in the sequence of soft to hard temper.
Usually a blackplate is produced by hot-rolling a low-carbon steel slab, cold-rolling a hot-rolled coil to a predetermined gauge, annealing, and skin-pass rolling. Tin plate is produced by tin-plating the blackplate. The annealing may be batch or continuous. Blackplate having a temper degree of T-1 to T-3 are conventionally produced by batch annealing. JIS also stipulates the production of blackplates having a temper degree of T-1 to T-3 to be by batch annealing, not continuous annealing. Since the heat cycles in continuous annealing are rapid heating, short-time holding, and rapid cooling, continuous annealing is conventionally applied for producing a blackplate having temper degree of T.sub.4 or more. Clearly, continuous annealing is advantageous over batch annealing in its high productivity, uniformity of quality, energy savings, labor savings, and a shorter delivery time. Accordingly, various methods have recently been considered for producing a soft blackplate having a temper degree of T-3 or less by continuous annealing.
It is well known that, for producing soft steel-sheets, including cold-rolled steel sheets, it is important to (a) coarsen the grain size, (b) decrease the solute carbon remaining in the matrix after annealing, (c) and decrease the solute nitrogen remaining in the matrix after annealing. In addition to these metallurgical factors, attention must be paid to the hardening amount in skin-pass rolling and subsequent steps. That is, with ordinary cold-rolled sheets, the final step for determining material properties is the skin-pass rolling. On the other hand, for example, with tin plate, the tin-plating step and the step of fusing the tin-layer for providing the surface lustre determine the final material properties, for example, the fusion inducing strain-aging hardening at a high temperature. For producing a soft blackplate for tin-plating by means of continuous annealing, it is therefore important not only to avoid hardening by grain-refinement and solid solution hardening by carbon and nitrogen, to soften the annealed sheet, but also to drastically decrease the solute carbon and nitrogen remaining in the annealed sheet to avoid strain-aging hardening during, for example, the fusion of the tin-layer.
In can production, soft blackplate steel sheets such as tin plate and tin-free steel (TFS) undergo blanking, painting, printing, and baking steps before and shaping. Since the steel is subjected, during the baking, to heat treatment of, for example, 180.degree. C. to 210.degree. C. for 10 to 20 minutes, severe aging is generated. The blackplate must, notwithstanding such aging withstand all the shaping work, i.e., drum-shaping, edge-working, flange-working, and seaming. In addition the worked surface of the steel sheet must not have folds due to aging, and there must be no fluting, i.e., buckling of the surface into polygonal lines during bending.
For preventing grain refinement (a), Japanese Examined Patent Publication (Kokoku) No. 55-48574 discloses, for example, to finish the hot-rolling at a low temperature of 700.degree. C. to Ar.sub.3, and Japanese Unexamined Patent Publication (Kokai) No. 58-27932, discloses to carry out the continuous annealing at a temperature of 680.degree. C. or more. For decreasing the solute carbon after the final annealing (b), Japanese Examined Patent Publication No. 55-48574 and Japanese Unexamined Patent Publication No. 58-27932 propose to carry out overaging treatment during the cooling from the soaking temperature. For decreasing the solute nitrogen, Japanese Unexamined Patent Publication No. 58-48574 and Japanese Unexamined Patent Publication No. 58-27932, for example, propose Al incorporation and Japanese Unexamined Patent Publication No. 58-187224 proposes addition of niobium.
The above proposals have been recently used for producing, by continuous annealing, a blackplates having a temper degree of T-3 or less. Nevertheless, they are only limitedly effective for stably producing blackplates. Therefore, decarburization by vacuum degassing is carried out at the steelmaking stage. In addition, the niobium incorporation is carried out such that the solute carbon and solute nitrogen are completely fixed. The cost increase due to the vacuum degassing and the addition of Nb offset the advantages of blackplates produced by conventional continuous annealing over backplates produced by batch-type annealing. Continuously annealed blackplates are currently being produced and marketed, but also still suffer from fluting. These problems should be eliminated so that can producers can carry out shaping after the painting, printing and baking without fluting.